FOC is all about stable flight. But you have to consider that the arrow's flight is not done until it comes to a complete stop. In other words, it is still in flight during the time it's penetrating that animal.

High FOC allows the arrow to remain more stable and resist deflection when it contacts ribs and such while passing through. A low FOC arrow can deflect off a rib, causing the shaft to have more contact with the surrounding tissues, causing extra drag and reducing the arrow's penetration. If the arrow can keep it's KE and momentum going in a straighter line - more stable flight path - while penetrating, it'll penetrate deeper.

That's my theory and I'm stickin' to it.